mtd: st_spi_fsm: only build for ARM

COMPILE_TEST allows us to build this driver on other arch'es. But not
all arch'es have the right I/O accessors -- particularly, x86 is missing
readsl() and writesl().

So just restrict this driver to ARCH_STI. It's still buildable for a
multiplatform ARM kernel, so it can get decent compile coverage.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>